Commercially important photochromic glasses have been derived from a glass family described in U.S. Pat. No. 3,208,860 (Armistead et al.). These glasses are characterized by having a precipitated microcrystalline silver halide phase.
U.S. Pat. Nos. 3,892,582 and 3,920,463 (Simms) describe a thermal reduction treatment for silver halide photochromic glasses that may be augmented by ultraviolet irradiation. The treatment is described as producing various fixed tints, primarily yellow, in the photochromic glasses.
The Simms patents disclose, by way of specific example, a glass having the following composition in approximate percent by weight:
______________________________________ SiO.sub.2 55.9 ZrO.sub.2 2.3 Al.sub.2 O.sub.3 9.0 Ag 0.16 B.sub.2 O.sub.3 16.2 Cl 0.29 Li.sub.2 O 2.65 Br 0.72 Na.sub.2 O 1.85 CuO 0.036 PbO 5.05 F 0.2 BaO 6.7 ______________________________________
It is stated that a permanent yellowish tint was imparted by firing this glass in a 5% H.sub.2 -95% N.sub.2 atmosphere for 8 minutes at a temperature of 450.degree. C.
A line of amber ophthalmic lenses was developed on the basis of the patent teachings. They faded slowly from the darkened state.
U.S. Pat. No. 4,190,451 (Hares et al.) describes a development of silver halide, photochromic glasses having improved darkening and fading rates. These glasses are also described as being susceptible to strengthening by either thermal or chemical tempering to enable lenses to pass the ball drop test prescribed by the FDA.
The latter procedure involves treatment in a molten salt bath to exchange sodium or potassium ions from the salt bath for lithium or sodium ions, respectively, from the glass. One such treatment involves exposure in a mixed salt bath for 16 hours at about 400.degree. C.
U.S. Pat. No. 4,240,836 (Borrelli et al.) describes a thermal reduction treatment which, when applied to the Hares et al. glasses, provides a variety of colors beyond the then known yellow. These colors are characterized by having absorption peaks in their transmittance curves to the right of a line extending from about 460 nm. at 0 transmittance to about 510 nm. at 100% transmittance. These peaks contrast with absorption peaks in prior glasses which generally occurred to the left of this line. The colors are produced by a reducing treatment at a temperature not over 450.degree. C. The treated glasses have improved photochromic properties and an attractive color in the darkened state.
Each of the previously available glasses provide certain features desired in a photochromic amber sunglass. However, no one of them manages to combine all of these in one glass.
Marketing research indicated that the color target for an amber photochromic sunglass lens in the faded state was one having a dominant wavelength near 577 nm, spectral purity between 35 and 40% and luminous transmittance of about 74%. Attempts to achieve that color target by hydrogen-firing the faster fading -451 glass (Code 8111 ) were not successful. Either the transmittance was too high at the desired chromaticity coordinates, or the spectral purity was too large at the desired transmittance. In either case, the lens appeared to be more yellow than amber.
An additional drawback for marketing acceptance was that the lens looked greenish in the darkened state. Hydrogen-firing a photochromic glass having a modified composition (Code 8122) yielded a more acceptable brownish color in the darkened state. However, that glass did not get dark enough in hot weather for use as a sunglass. Also a desired faded color was not achieved.
A pleasing, though somewhat dark, faded color could be made using a reduced photochromic glass which also employed a fixed tint colorant. That glass had acceptable darkened color, but did not fade as rapidly as was desired.
There has remained a need for a photochromic sunglass that combines good photochromic properties with an amber fixed tint having a more acceptable appearance in both the faded and darkened states. In particular, the faded glass should be a true amber, rather than a bright yellow. Also, the darkened glass should have a reddish brown tint, rather than a greenish cast.